1. Field of the Invention
The invention relates to a fuse, and more particularly to a quick-break fuse having at least one fuse link. Such fuses are used, preferably in the medium-voltage range, although in AN everyday language, they are called high-voltage or high-voltage quick-break fuses.
2. Description of the Prior Art
A fuse is a device which, by melting of one or more of its parts intended and designed for this purpose, opens the circuit, in which it is inserted, by interrupting the current if the current exceeds a predetermined value for a sufficiently long period of time. The fusible element or fuse link is intended to melt away under the influence of a current which exceeds a particular value for a particular length of time.
High-voltage quick-break fuses, as a rule, consist of a tubular ceramic body with conductive caps at both ends, between which, in the interior of the tube, fusible elements which are electrically connected to the caps are fixed on a ceramic insulting body. The whole fusible element structure is embedded in a bed of silica sand which serves to cool the arc, whereby a quick-break capacity of the fuses is obtained.
The fuse links concerned here are fuse links acting in a current-limiting and selective manner which are automatically tripped by the melting of a material to protect equipment, preferably, in medium-voltage networks of power supply plants and industry.
The brochure "High-voltage quick-break fuse links for indoor and outdoor switch bear" of the applicants' assignee discloses such fuses operating on the principle described above, wherein fusible elements arranged parallel to one another are placed in grooved webs of a star-shaped fusible element carrier and are thereby fixed exactly in position. This form of construction leads to quick-break capacity, since through this kind of arrangement of the fusible elements and the distinctive shape of the grooved webs of the star-shaped fusible element carrier, a system of series-connected chambers is formed. Upon interruption of short-circuits, a partial arc is formed in each of these chambers, which is precisely controlled by constrictions in the fusible element so that on evaporation of the fusible element a multiple disconnection occurs.
However, the disadvantage of the known fuse consists in that for physical reasons, it cannot interrupt a current below the minimum cut-off current. This is due to long melting times in the case of small excess currents and the accompanying excessive heating up.
A distinction is, therefore, made between sub-range and full-range fuses. The latter are capable of interrupting any current up to the rated cut-off current. Fuses of this kind are known, for example, from German patent documents 24 12 688 B2 and 32 37 326 A1, which propose to fit two electrically series-connected systems in one fuse link, with one of the systems corresponding to the current-limiting sub-range fuse, while a second system, which comprises fusible elements of tin housed in heat-resistant silicone tubes serves to interrupt the excess current. In case of an overload, the tin melts and flows under pressure out of the tubes into the silica sand, so that a breaking gap results.
In order to achieve complete overload protection in the case of sub-range fuses, these fuses are very often fitted in combination with an instantaneous fuse striker. In this case, a retaining wire located within the fuse releases the striker pin in case of overloads and effects a three-phase cut-off by way of the switch disconnector which follows. This construction is however regarded by many users as a technically and economically unsatisfactory solution of the problem, since with the three-phase cut-off, the network supply cannot be further maintained through the switch load-break which is also required, and the construction of the tripping mechanism is relatively complicated.
It is preferred to use full-range fuses, which eliminate the above-mentioned disadvantages since the cutting-off of the overload and short-circuit currents is undertaken directly by the fuse link. The known form of construction described above comprises connection in series of two switching systems which operate on different principles, and the characteristic curve therefore results from the superposition of the characteristics of the two switching systems.
It has now been found that cut-offs at the point of the intersection of the two characteristic curves, i.e., in the transition region between short-circuit and overload regions, cannot always be reliably controlled. Switching failures can occur in the vicinity of the point of intersection of the two characteristic curves if the current is too large for the quenching system but is too low for the silver fusible element. Since the overall lengths of the high-voltage quick-break fuses are prescribed by a standard, the two switching systems are divided in the full-range fuses so that about one-third is provided for the overload region and two-third for the short-circuit region.
Because of the smaller space available, the switching capacity of the switching system responsible for the short-circuit currents is reduced, and the minimum cut-off current is likewise substantially higher. This means in addition that the dielectric strength after successful cut-off has to be achieved on a shorter region of the fuse. This is serious, since it can result in restriking.
In addition, full range fuses of this construction cannot be built in all sizes. Thus for technical reasons these fuses cannot be realized for the small 20 kV fuse range.
A quick-break fuse having at least one fuse-link (the fusible element) and means for mechanically opening the fuse-link or links by means of a mechanical actuator connected to an energy accumulator and having a tripping wire is in fact known from U.S. Pat. No. 4 189 694. In this fuse, however, the tripping wire serves to supply a movement initiator with current in the event of a fault. This wire must thus in no circumstances be broken. Moreover, this fuse is not suitable for the low excess current region, since it does not react to the heating up that occurs as a result of the excess currents, but at best only when the first arc is formed.
Accordingly, an object of the invention is providing, in place of the known full-range fuse, a general-purpose fuse which is capable of interrupting any current, up to the rated cut-off current, in a reliable and detectable manner. Another object is to achieve, without restriction of the response characteristics in the short-circuit region, all the dimensions possible according to the standard, and moreover, to avoid combinations with switching devices in order to achieve an economical and inexpensive design.